7 replies to this topic

[nawillia]

This is a multi-part post to document the DIY conversion of my CGE Pro mount to OnStep.

 

I purchased a used CGE Pro locally that had developed the “ludicrous speed” issue where the RA motor ran at an extreme speed as soon as the mount was powered on. I read about various fixes and theories about the cause of the issue on here, but did not find any clear consensus on the cause (maybe a few different causes). I have also been reading about OnStep for the last couple of years and decided I would like to undertake a conversion to replace the original servo motor drive with stepper motors. There is a “bolt-on” OnStep conversion kit available for the CGE Pro that seems to be highly regarded, though I opted to save some money by implementing the conversion myself.

 

The OnStep documentation was initially intimidating (I don’t have a background in electronics), but I joined the group and read through the various wiki pages a few times and got the hang of what was involved (what a stepper motor driver is, etc.). I also found a few write-ups from folks that have previously converted the CGE Pro that was extremely helpful in figuring out the gear reductions, etc. After spending some time with the configuration spreadsheet, I decided the MaxPCB4 would be a good option for allowing the types of slew rates and stepper resolution that I would like to achieve for astrophotography.

[nawillia]

Having built a “MyFocuserPro2” focuser controller last year, I had a reasonable understanding of what would be involved (lots of soldering, loading firmware). My experience from the focuser controller is that all of the individual electrical components are relatively inexpensive, but require you to purchase many extra parts. Instead of sourcing all of these myself, I ended up purchasing a pre-assembled PCB. This cost me slightly under $100 shipped and did not include the Teensy microcontroller board that I bought separately on Amazon for about $40 and simply plugs into the PCB. The assembled PCB came with two LV8729 stepper motor drivers, though I separately purchased some TMC5160 drivers that allow higher amperage steppers. In hindsight, the drivers that came with the assembled PCB seem to be working fine and I could have saved about $30 by not buying the new drivers, but I will probably install them anyway.

 

This is a photo of the assembled PCB with the Teensy board installed and all within the 3d printed controller case.

 

[nawillia]

The steppers I chose after reviewing the spreadsheet are Nema17 400 step 2 amp motors. I was a little concerned about the torque of these motors compared to the “coarser” 200 step version, but the 400 step motors seem to work fine in my limited testing so far (although I have not yet loaded the mount anywhere close to its 90 lbs capacity). I also briefly considered the Nema23 options that can offer more torque, but I think would be more challenging to fit within the space available on the mount. The gear reduction is 5:1 from a 12 tooth GT2 pulley on the motor shaft to a 60 tooth pulley on the worm shaft.

 

The Nema17 motors are a bit physically larger than the servo motors original to the mount. As a result, the options are to either do a bit of cutting or drilling on the original mounting bracket, or find a way to bolt on an adapter. I opted for the second choice because I wanted to not make any permanent modifications in case I need to revert the changes back for some reason. I designed and 3d printed a new motor bracket in carbon fiber PETG filament. I did some research and found that I can upload the CAD file online and have the parts manufactured from aluminum for around $80 for the pair – I figured I would try the 3d printed version first to see if they are rigid enough before committing to the additional expense.

 

This is what the bracket looks like and a photo of the motor attached to the mount.

 

 

[nawillia]

The motors stick out too far to use the original aluminum motor housing “boxes.” I designed new larger boxes that accommodate the new motors and brackets. It took a couple of iterations to get these to fit correctly. I still had to take a dremel to the “final” version to remove some plastic from near the new worm pulley to prevent rubbing on the lid portion. I decided I wanted the cable connections to be modular while I figured out the design, so printed the box with holes in the side that I can screw various plates with the electrical connectors to. The reason for circular holes is to avoid large “bridges” when 3d printing as they don’t print very cleanly for me. I would prefer these holes to be very slightly larger, but I think they will work ok as is.

 

This is what the housing boxes look like and a photo of the motor housing “boxes” attached to the mount.

 

 

 

A photo of the GX16 connectors on the 3d printed OnStep controller box. I had to modify the design on the OnStep site a bit to get the GX16 connectors for the motors to fit.

 

Edited by nawillia, 09 November 2024 - 02:43 PM.

[nawillia]

After a run of really nice, clear weather in early October, it has been back to mostly clouds in middle Tennessee the last couple of weeks. Last night there was a small window in the forecast with less cloud cover, so I spent some time getting everything set up to finally do some real testing.

 

This is a photo of the mount with my recently assembled dual imaging rig.

 

[nawillia]

I spent a long time last night troubleshooting a new computer configuration that involved a NUC mini PC connected to a raspberry pi running Kstars/Ekos all accessed via a remote desktop connection from indoors. I will probably keep troubleshooting this, but eventually turned off the mini PC and connected directly to the raspberry pi. I think all of equipment needed for the dual rigs is taxing on the pi, so probably not a good long-term solution, but will figure that out another day. Finally got everything running on the pi and was able to experiment with the relatively new dual-imaging options in Ekos (seemed to work very well).

 

I was very pleased with the guiding. My guiding RMS was about 0.5-0.6” for the 90 minutes I had before clouds completely covered the sky. I will have to use this a few more times to continue testing, but in general I am really happy with the initial results. I have a larger/longer OTA I’d also like to try on the mount soon that was pushing the imaging capacity on the Atlas Pro but should be well within the capacity of the CGE Pro.

 

This is a sample guide log from Ekos.

 

[nawillia]

Next steps are to figure out how to implement the home switches and maybe PEC functionality from the original mount. I made provisions for a 3.5 mm audio jack connector to the DEC housing and the DB15 connector to the RA housing. I’m not sure what getting the original switches to work with OnStep entails, hopefully not a whole lot more than ensuring they are wired correctly and enabling it in the OnStep configuration file.

 

I also need to figure out a way to adapt the mount to my cinder block pier. The mount is physically very large – while possible to move the components separately in mostly manageable parts, I’d prefer to not have to do that each time I want to use it.

[nawillia]

Received the TMC 5160 stepper drivers ("Chaofan" brand)  this week and got them installed and updated the OnStepX configuration file. Based on what I read in the OnStep group, I initially set the "GOTO" current to about 2000 mA in the configuration file for my 2A Nema17 motors. This seemed like it was too high and the motors were stalling part way through the test slews. I lowered this value a few times and settled on 1150 mA. This has seemed to worked ok during limited testing, will have to wait to see how they perform when the mount is fully loaded.

 

Also got the mount attached to my "Todmorden" cinder block pier. Would like to find a better pier adapter, but think the temporary attachment should be ok for now. Hope to have a few hours of clear sky tonight to do a bit more testing. Had a couple of clear nights earlier this week and was really pleased with the performance of the mount.

 

Also ordered some M12x1.5 "fine" thread stainless bolts on eBay to replace a couple of missing knobs that lock the altitude adjustment. This does not seem to be a very common thread pitch in the US and the ones I ordered were shipped from China. I designed and 3D printed some knobs to try to generally match the style of the other knobs on the mount. You may be able to see the new knobs in the attached photo (they aren't an exact match to the originals, but close enough).